A general handover and initial network entry process will hereinafter be described in detail.
FIG. 1 is a flow chart illustrating a method for performing a handover and initial network entry process.
Referring to FIG. 1, a mobile station (MS) selects a cell to enter the initial network or to perform the handover at steps S101 and S102. During this cell selection, the mobile station (MS) performs scanning or ranging for at least one base station in order to search for an appropriate base station for either a network connection or a handover. The mobile station (MS) may schedule a scan period or a sleep period to determine whether or not the mobile station (MS) can enter an initial network for the base station or a target base station.
When the mobile station (MS) enters the initial network, it can be synchronized with a serving base station (SBS) simultaneously while acquiring downlink parameters from the serving base station (SBS) at step S103. In this case, the SBS may provide a service over the network desired by the MS. The MS establishes synchronization with the SBS, and then acquires uplink parameters for the SBS at step S104. The MS performs the ranging process with the SBS, and adjusts the uplink parameters along with the SBS at step S105. By the above-mentioned steps, the MS and the SBS form basic functions for communication at step S106. The SBS authorizes the mobile station (MS) and exchanges keys with this MS at step S107. So, the MS is registered in the SBS at step S108, and establishes an IP connection to the SBS at step S109.
The SBS transmits operation parameters to the MS, such that it can communicate with the MS at step S110. A connection between the MS and the SBS is established at step S111, such that the MS and the SBS can carry out normal operations at step S112. The MS can continuously search for a neighboring base station while the SBS performs normal operations at step S113. As the MS becomes distant from the SBS while in motion, a quality of service (QoS) provided from the SBS is gradually lowered, such that the mobile station (MS) must continuously search for the neighboring base station capable of providing better services. In this case, the neighboring base station providing a service better than that of the SBS is called a target base station (TBS), and the MS searches for the TBS, such that this MS can be handed over to the searched TBS.
Generally, the handover is carried out when the MS moves from the serving base station (SBS) to the target base station (TBS). Namely, the handover is carried out when a radio interface, service flow, and a network access point of the MS are shifted from the SBS to the TBS. If the MS, the SBS, or a network administrator decides the handover at step S114, the handover is started.
The MS selects the TBS at step S115, can establish synchronization with the TBS, and can also acquire downlink parameters from the TBS at step S116. The MS acquires uplink parameters from the TBS at step S117, performs the ranging process with the TBS, and adjusts the uplink parameters along with the TBS at step S118. In this case, if the MS has received an NBR-ADV message including an identifier of the TBS, a frequency, and uplink/downlink channel descriptors (UCD/DCD), the scanning process and the synchronization process can be simplified. If the TBS receives a handover notification message from the SBS over a backbone network, a non-competitive initial ranging opportunity may be provided to a UL-MAP.
By the above-mentioned procedure, the MS and the TBS form basic functions at step S119. The MS and the TBS perform the ranging process to start re-entering the network. Also, the MS is re-registered in the TBS, and establishes a re-connection to the TBS at step S120. So, the MS is registered in the TBS at step S121, and an IP connection from the TBS is re-established in the MS at step S122. As a result, the TBS serves as the SBS, such that it can provide the MS with necessary services.
The handover of FIG. 1 will hereinafter be described in detail. The MS re-selects the cell on the basis of neighboring base station information acquired by the scanning, and is handed over from the SBS to the TBS. Therefore, the MS establishes synchronization with the TBS and performs the ranging process with the TBS. Thereafter, the TBS performs reauthorization for the MS. Herein, the TBS can request information of the MS from the SBS over a backbone network.
The handover and network reentry process can be greatly simplified according to MS-associated information owned by the TBS. Some network entry processes may be omitted according to an amount of MS-associated information owned by the TBS.
If the MS moves from a cell area of a current SBS to a cell area of a TBS, it performs a handover to the TBS, and performs a ranging process.
The term ‘Ranging’ is indicative of a set of processes for maintaining an RF communication connection quality (especially, synchronization) between the base station and the MS in an IEEE 802.16 or a portable Internet. In case of a downlink for an OFDMA-based multiple accesses, the base station transmits several mobile stations at one reference time, such that the above ranging process has no problems. However, in case of an uplink, several mobile stations transmit different signals, such that they may have different propagation delays in the uplink. Therefore, the base station and the mobile station perform the ranging process using an additional timing synchronization method for coping with different propagation delays. That is, the ranging process is needed for several mobile stations to correctly adjust synchronization of a transmission (Tx) time.
There are a variety of methods which can acquire uplink synchronization between the mobile station and the base station simultaneously while performing a power control between them, for example, an initial ranging method, a periodic ranging method, and a hand-off (HO) ranging method. In addition, there may also be used a bandwidth request (BR) ranging method for allowing the mobile station to request a bandwidth from the base station.
If the mobile station initially enters an area of the base station (e.g., if the mobile station is powered on or is shifted from an idle mode to an effective or valid mode), the initial ranging process is used for the initial network entry and combination process. The periodic ranging is periodically transmitted such that the mobile station is able to track synchronization of the base station. The hand-off (HO) ranging is carried out by the mobile station such that the mobile station is able to establish synchronization with the target base station (TBS) during a handoff (or a handover) time. The bandwidth request (BR) ranging is transmitted such that the mobile station is able to request an uplink allocation from the base station. The BR ranging is transmitted from only the mobile station which has established synchronization with a system.
If the base station receives a ranging request message from the mobile station, it generates a connection identifier (CID) of the corresponding mobile station. Thereafter, the base station performs a variety of processes on the mobile station, for example, a renegotiation process, a reauthentication process, and a reregistration process.